Such droplets are used in several technical fields. In each field, the methods of forming droplets are different.
A first technical field relates to on-chip laboratory applications or other biotechnologies. In this field, a first approach consists in using a device having at least one microchannel in which a first fluid flows, also known as a “carrier” fluid, which microchannel leads perpendicularly into at least one second microchannel in which a second fluid flows that is not miscible with the first fluid. The first fluid (generally oil) shears the second fluid (generally water in biological applications) so as to form droplets of the second fluid, which droplets are then transported by the first fluid. The flow rates of the two fluids and the shapes of the microchannels are adjusted so as to form droplets of a desired size at a desired rate, where size and rate also depend on the viscosities of the two fluids.
Devices of that type necessarily include forcing means such as a pump to cause the two fluids to flow. Since droplet size is a function of the flow rate of each fluid, it is necessary to adjust the fluid flow rates accurately, thereby making such devices difficult to use.
For example, document US 2006/0051329 describes a device for encapsulating droplets containing cells, the device having both a first channel for delivering a flow of fluid containing the cells, which channel presents a flare in its downstream portion (see FIG. 9), and also a second channel conveying a flow of oil that crosses the first channel perpendicularly, the oil flow shearing the flow of fluid containing the cells so as to form droplets. A similar device is described in the document “Controlled microfluidic encapsulation of cells, proteins, and microbeads in lipid vesicles”, by Yung-Chieh Tan et al., Journal of the American Chemical Society, Vol. 128, No. 17, Apr. 5, 2006, pp. 5656-5658.
Document WO 2009/048532 also describes a device for forming droplets, which device comprises a first channel for delivering gas into which two opposite side channels lead to deliver water in such a manner as to form bubbles of gas surrounded by water, and in which two other side channels lead to deliver oil enabling the bubbles to be encapsulated. The bubbles are formed by shearing a flow of a first fluid (gas) with the help of a flow of another fluid (water).
Other devices for forming droplets by shearing a fluid flow are disclosed by the following documents: WO 2010/033200; WO 2007/133710; “Holographic control of droplet microfluidics”, by M-L Cordero et al., Proceedings of the SPIE, Vol. 7038, Aug. 10, 2008, pp. 70381J-1; “Electrically initiated upstream coalescence cascade of droplets in a microfluidic flow”, by Michele Zagnoni, et al., Physical Review E., Vol. 80, No. 4, October 2009, pp. 046303-1-046303-9; and “Thermocapillary manipulation of droplets using holographic beam shaping: microfluidic pin ball”, by M-L Cordero et al., Applied Physics Letters, Vol. 93, No. 3, Jul. 24, 2008, p. 34107.
Document US 2009/0098168 discloses a device for forming droplets, which device has a channel for delivering a fluid flow that leads into an expansion nozzle via an orifice. The nozzle has two diverging walls and contains a fluid that is different from the fluid flowing in the channel upstream from the orifice. Droplets are formed because of hydrodynamic pinching at the orifice, while the diverging walls ensure that the emulsion is uniform. The system forces a central stream of a dispersed phase and two side sheath flows through the orifice into a second chamber, with the convergence of the flow surrounding the liquid serving to fraction the thread at the orifice.
A second approach is that of so-called “digital” microfluidics, in which the droplets are typically formed by electrowetting, by applying different electrical voltages to different portions of the droplets.
Droplets formed using that technique are of a size that is much greater than that of nanodroplets or microdroplets. That technique also raises the problem of contamination between droplets and of droplet evaporation.
Finally, there exist several approaches for producing droplets on demand by quickly ejecting liquid through a needle or a hole with the help of devices that are often similar to ink jet printer systems, producing droplets that impact against a surface with high energy and generate splashes. Those devices further require expensive technical means such as high voltage sources or precision motors.
A second technical field relates to materials science, in which several approaches have been developed in order to produce foams or emulsions, and thus populations of bubbles or droplets. Applications are varied and relate in particular to the food industry and the cosmetics industry.
Other approaches consist in encapsulating droplets in other droplets. For example, a water droplet may be encapsulated in an oil droplet, which is itself contained in water. All of those approaches require the use of expensive forcing means that are difficult to implement.
In addition, and in general, it is desired to increase the rates at which droplets are produced while guaranteeing that droplets or bubbles are obtained in monodisperse form, i.e. presenting size that is constant and controlled.